Modern satellite communication systems provide a robust and reliable infrastructure to distribute voice, data, and video signals for global exchange and broadcast of information. These satellite communication systems have emerged as a viable option to terrestrial communication systems for carrying data traffic such as Internet traffic. A typical satellite Internet system comprises subscriber terminals, a satellite, a ground station, and connectivity to the internet. Communication in such a system occurs along two links: 1) an uplink (or inroute) from a subscriber terminal to the satellite to the ground station to the gateway to the Internet; and 2) a downlink (or outroute) from the internet to the gateway to the ground station to the satellite to the subscriber terminal.
Very Small Aperture Terminals (VSATs) are commonly used as subscriber terminals for transmitting and receiving wireless signals on phase modulated carriers in satellite communications systems. On the transmission (inroute) side, a VSAT includes an indoor unit for modulating a signal with information, a block upconverter (BUC) for upconverting the frequency band of the signal, and a parabolic dish for focusing and transmitting the upconverted signal to a satellite. FIG. 1 illustrates a conventional implementation of transmission circuitry 120 in an indoor unit. Transmission circuitry 120 includes a forward error correction (FEC) block 121, an application-specific integrated circuit (ASIC) modulator 122, and digital-to-analog (DAC) converter 127. As illustrated, the conventional VSAT encodes bits into bursts on an ASIC modulator 122 based on a designated FEC rate and a start of burst (SOB) time determined by a modulation SOB timing unit 124.
More particularly, burst formatter 123 formats a signal into a burst format (e.g., 8PSK-burst format) by inserting pilot symbols, unique words (UW), a payload, and ramp and guard symbols. Symbol mapper 125 then performs the functions of bit-to-symbol conversion, burst segment scaling, signal spreading, and continuous wave generation (CW). Transmit filter circuitry 126 subsequently shapes and upsamples the signal using filters such as an RRC filter, a polyphase filter, a ramper, a phase-locked loop (PLL) precompensation filter, a quadrature modulator, etc. As illustrated, ASIC modulator 122 directly outputs the modulated data signal (i.e., I and Q data channels) to a DAC converter 127 that then feeds the signal to a BUC.
Current implementations of Jupiter VSAT terminals are limited to transmission symbol rates of 8 Msps or lower due to the conventional transmit filter design implementation. The current design is problematic as it is unable to take advantage of interfacing with higher power linear block upconverters (BUC) capable of supporting an inroute satellite link with greater data transmission speeds.